Mark external families and synthesise unresolved-fam-instance nodes

Mark external families and synthesise unresolved-fam-instance nodes

External families (referenced via FamilyApp but absent from
pdTypeFamilies) now render as grey dashed diamonds, matching the
external-class styling.

When a context predicate or unmatched superclass mentions a
FamilyApp that no real fam-instance can resolve — typically because
the family is external — synthesise a placeholder fam-instance node
"Family args = ?" for each distinct use site. The chain then reads:

    SigDSIGN ──► SigDSIGN Ed448DSIGN = ?  ╶╶►  NoThunks (SigDSIGN Ed448DSIGN)

(the dashed leg is the fam-resolves edge to the originating predicate
node). Side panel for these placeholders explains they're use sites,
not equations. Help legend grows two new rows: external family +
unresolved fam-instance.

addFamilyLinksFromArgs gained two parameters (boundTvs, originPredId)
to render the placeholder labels in the right tyvar context and to
chain placeholders back to the predicate that needed them. The
superclass call site reorders so the unmatched-pred node id is
available before the call.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>